Sound localization in the alligator

Abstract

In early tetrapods, it is assumed that the tympana were acoustically coupled through the pharynx and therefore inherently directional, acting as pressure difference receivers. The later closure of the middle ear cavity in turtles, archosaurs, and mammals is a derived condition, and would have changed the ear by decoupling the tympana. Isolation of the middle ears would then have led to selection for structural and neural strategies to compute sound source localization in both archosaurs and mammalian ancestors. In the archosaurs (birds and crocodilians) the presence of air spaces in the skull provided connections between the ears that have been exploited to improve directional hearing, while neural circuits mediating sound localization are well developed. In this review, we will focus primarily on directional hearing in crocodilians, where vocalization and sound localization are thought to be ecologically important, and indicate important issues still awaiting resolution.

Evolution of Sound Source Localization Circuits in the Nonmammalian Vertebrate Brainstem

“…The brain stem nuclei in birds and crocodilians also appear proportionally larger than those of lizards and turtles [Bierman and Carr, 2015], which may reflect the greater range of neural computations taking place there.…”

Coupled ears in lizards and crocodilians

“…Lizard ears are coupled (Christensen-Dalsgaard 2005; Christensen-Dalsgaard and Manley 2005; Christensen-Dalsgaard and Manley 2008; Christensen-Dalsgaard et al 2011) and in consequence all auditory responses in their brains should be directional, without a requirement for computation of sound source location. Crocodilians and birds have partially coupled ears (Bierman and Carr 2015), and the effects of this coupling on CNS computation of sound source direction vary with frequency. Coupling may improve the processing of low frequency directional signals in archosaurs, while higher frequency signals are progressively uncoupled.…”

“…7B). Thus crocodilian NL neurons act as coincidence detectors, and employ similar algorithms for ITD detection to birds (Bierman and Carr 2015). Nevertheless, the range of best ITDs represented in alligator NL was much larger than in birds, however, and extended from 0 to 1000 μs contralateral, with a median ITD of 450 μs (Fig.…”

Sound Localization Strategies in Three Predators

“…We focus on barn owl localization behavior, since there are no psychophysical measures from alligators and lizards, although there are observations of localization behavior in each group [Bierman and Carr, 2015; Sakaluk and Belwood, 1984]. Minimum resolvable angles have been studied in a number of bird species, with thresholds ranging from 2–3° in the barn owl, saw-whet owl and marsh hawk to over 100° in the zebra finch [Klump, 2000; Rice, 1982; Bala and Takahashi, 2000; Bala et al, 2003].…”

“…This coupling generates increased directional cues and may modify the available ITDs. Such air filled cavities or sinuses appear to have evolved independently in crocodilians and other archosaurs, including dinosaurs and their avian descendants [Dufeau, 2011, reviewed in Bierman and Carr, 2015]. Sound transmission through these sinuses would allow the eardrums of alligators to act to some degree like pressure-difference receivers.…”